The proposed objectives are: To ascertain whether classical immunological tolerance to avian oncornavirus antigens and virus-induced surface antigens exists at the humoral or cellular levels; to determine the specificity of this tolerance; To isolate virus-induced surface antigen and to study its role in the evocation and operation of immunity to avian oncornaviruses. BIBLIOGRAPHIC REFERENCE: Meyers, P. Antibody Response to Related Leukosis Viruses in Chickens Tolerant to an Avian Leukosis Virus, J. Natl. Cancer Inst. 56, 1976 (In Press).